To quote from PSGroove: "I have just tested this and it is stable, bug free, and FAST (11MB/sec for me). Also thanks blackb0x for the shout out, it seems our site is becoming quite popular :)"

PlayStation 3 FTP Server 1.0b:

Well since no one has released a bug free, speedy, easy to use FTP server we decided to take the reigns on it. Introducing FTP Server 1.0b, it doesn't crash, it doesn't byte shift, and most importantly, it doesn't suck. Oh, and its threaded for speedy transfers :)

Instructions:

Install the pkg to your ps3 (you should know how to do this by now) then run blackb0x FTP. You can now connect to your PS3 via the PC FTP client of your choice. Just fill in your PS3?s ip, port 21, and leave username, and password blank. To quit the app either press X, or quit via ingame XMB.